Performance problem PBO + glCompressedTexSubImage3D + Texture Array

Hello there,

i've a problem with updating a texture array with compressed data per glCompressedTexSubImage3D.

I have an PBO which stores the compressed data, which i want to use to update a 2D array texture.
Unfortunately the updating process is very slow and need 100 ms.
The upload process to the GPU is already finished when i call glCompressedTexSubImage3D, because i use
fences to check for that.

What's odd is the fact, that this problem does only occur if i create an array texture with more then one layer.
With one layer it works like a charm. If i create an array texture with let's say 2 layers, the problem even occurs for the first layer.
My driver version is 331.38 and i use linux.

I hope someone can tell me what i am doing wrong or at least confirm that this is a driver bug.

Edit: Oh i forgot to mention that i'm using a Debug context and that no errors are thrown.